Gtk3 ui: Add libgtk3ui as a separate build component

chrome/browser/ui/BUILD.gn

Index: chrome/browser/ui/BUILD.gn
diff --git a/chrome/browser/ui/BUILD.gn b/chrome/browser/ui/BUILD.gn
index 97d00e08b8ce1d6be50fe101c2ab361a4a58c71e..6c7a764e2fdd55f92a5f2e405f9bd974c8c73b50 100644
--- a/chrome/browser/ui/BUILD.gn
+++ b/chrome/browser/ui/BUILD.gn
@@ -1891,11 +1891,13 @@ split_static_library("ui") {
     deps += [ "//build/linux/libgio" ]
   }
   if (use_aura && !use_ozone && is_desktop_linux) {
-    deps += [
-      # gtk2 is the only component that can interact with gtk2 in our new
-      # world.
-      "//chrome/browser/ui/libgtk2ui",
-    ]
+    # gtk{2,3} is the only component that can interact with gtk in our new
+    # world.

[Dirk Pranke, 2016/10/25 23:56:10]

Nit: I'd rewrite this to "these are the only components that can interact with
gtk".

[Tom (Use chromium acct), 2016/10/26 18:20:01]

Done.

+    if (use_gtk3) {
+      deps += [ "//chrome/browser/ui/libgtk2ui:libgtk3ui" ]
+    } else {
+      deps += [ "//chrome/browser/ui/libgtk2ui:libgtk2ui" ]
+    }

[Dirk Pranke, 2016/10/25 23:56:10]

You eventually want to be building and depending on both, right? Because if you
don't want that, we should structure the targets a bit differently.

[Tom (Use chromium acct), 2016/10/26 18:20:01]

We'll want to build both during our gtk3 transition, but I think after that is
over, we would only need to build the gtk3 version.

   }
   if (is_win || is_mac || is_desktop_linux) {
     sources += [